1. Field of the Invention
The present invention relates to a novel catalyst useful for producing chlorine by oxidizing hydrogen chloride, a method for producing the catalyst and a method for producing chlorine by using the catalyst. More particularly, the present invention relates to a cerous chloride-chromic oxide (CeCl.sub.3 --Cr.sub.2 O.sub.3) catalyst for producing chlorine, its method of preparation and a method for efficiently producing chlorine by using the catalyst.
2. Description of the Prior Art
Chlorine, an important material in the chemical industries, is used in a large quantity to prepare organochlorine compounds and in phosgenation reaction. Of the chlorine used, only a portion is utilized for producing products, and the rest is eliminated as hydrogen chloride gas. The hydrogen chloride gas by-product, is recovered in a useful form of 35% hydrochloric acid by absorbing it in water. However, since excess hydrochloric acid should be neutralized and discarded, it is disadvantageous from an economic point of view. In addition, when the neutralization is incomplete, it induces serious pollution problems.
Recently, research and development efforts have been directed to the utilization of the by-product hydrogen chloride by oxidation to chlorine, in order to solve the problem of surplus hydrogen chloride and to cope with the increasing demand for chlorine.
Since Deacon invented a copper catalyst for oxidizing hydrogen chloride to chlorine in 1868, a variety of catalysts, such as iron catalysts and chromium catalysts, have been reported. For example, the use of copper catalysts which were modified from Deacon catalysts, is suggested in U.S. Pat. Nos. 2,418,930, 2,418,931 and 4,119,705, and Japanese Patent Laid-Open Publication No. Sho.53-125,989. However, the copper catalysts generally have a short life time, because the catalytic reactions should be carried out at high temperature of at least 400.degree. C. An iron catalyst is disclosed in U.S. Pat. No. 2,577,808, but this catalyst has a short life time because of the high reaction temperature, i.e., at least 400.degree. C.
A chromium catalyst is proposed in U.K. Patent No. 676,667, as a substituent for the copper or iron catalyst. The catalyst is prepared by dissolving chromium trioxide (CrO.sub.3) in water, drying and reacting with hydrogen at 200.degree. C. for 4 hours to give chromic oxide (Cr.sub.2 O.sub.3). The conversion of hydrogen chloride to chlorine using the catalyst thus obtained ranges from about 60 to about 7%. The catalyst, however, also has a short life time since the catalytic reaction for producing chlorine is carried out at high temperature of at least 400.degree. C.
Many patents, for example, Korean Patent Publication Nos. 86-4802, 90-79 and 90-2545, and Japanese Patent Laid-Open Publication Nos. Sho. 62-153103, 62-191403, 62-241805 and 62-275001, use chromic oxide (Cr.sub.2 O.sub.3) as catalysts for producing chlorine. The catalysts are prepare by dissolving chromium nitrate in de-ionized water, reacting it with a basic compound such as ammonia water, and calcining the precipitate thus obtained.
The chromium oxide catalysts mentioned in those patents are similar to MT-Chloro (trade name, commercially available from Mitsui Toatsu Chemical., Japan), which is prepared by dissolving chromic nitrate nonahydrate (Cr(NO.sub.3).sub.3. 9H.sub.2 O) or chromium trioxide (CrO.sub.3) in de-ionized water, adding 25% ammonia water dropwise to the solution, and subjecting the precipitate obtained through a series of treatments, for example, filtering, washing, drying and calcining, to give chromic oxide (Cr.sub.2 O.sub.3), and impregnating it on SiO.sub.2. The conversion of hydrogen chloride by MT-Chloro catalyst is in a range of 63 to 79%.
In processes for producing chlorine by use of the chromium catalysts reported up to now, the chromium catalysts are prepared by treating aqueous chromium salt with an alkaline earth hydroxide or an aqueous alkaline carbonate solution, and applying to the precipitate thus obtained a series of treatments including filtration, washing, drying and calcining. However, the obtained catalyst still has low activity and requires relatively high reaction temperature, causing the problem of a short life time.